The fluorescence emission of carbon dots (CDs) of ∼4 nm diameter was studied in a large number of solvents ranging from the weakly polar 1,4-dioxane to the strongly polar and protic water and formamide. The emission spectra in all solvents comprise of two major components, attributed to two distinct populations of emission centers, the edge-states at the periphery of the sp²-hybridized CD core and the surface fluorophores. Emission from both these centers are sensitive to solvent polarity and H-bonding capacity and undergo prominent redshift with solvent E_T(30) polarity parameter, which reveals a comparable polar character of these centers and their strong interaction with solvent molecules. However, between these two centers, the emission of surface fluorophores is more drastically quenched at high E_T(30) solvents, suggesting the opening up of additional nonradiative relaxation channels. This distinct solvatochromic response of these two emission centers also enables the CDs to perform as ratiometric probes for solvent polarity.

中文翻译：

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碳点的溶剂变色响应：溶剂与不同类型的排放中心相互作用的证据

在从弱极性的1,4-二恶烷到强极性的质子水和甲酰胺等大量溶剂中研究了直径约4 nm的碳点（CD）的荧光发射。所有溶剂中的发射光谱均包含两个主要成分，这归因于两个不同的发射中心种群，sp ^2-杂交的CD核外围的边缘态和表面荧光团。这两个中心的发射都对溶剂极性和氢键容量敏感，并且在溶剂E _T下会发生明显的红移（30）极性参数，它揭示了这些中心可比的极性特征以及它们与溶剂分子的强相互作用。然而，在这两个中心之间，表面荧光团的发射在高E _T（30）溶剂中被更彻底地淬灭，这表明开放了其他非辐射弛豫通道。这两个发射中心的明显溶剂变色反应也使CD能够用作溶剂极性的比例探针。